Projection televisions (PTVs) are well known. In general, a projection television set or projector includes three cathode ray tubes (CRTs), corresponding to the primary colors, red, blue and green. Associated with each of the CRTs is a projection lens assembly, which is comprised of a plurality of lens elements. Overall, the function of the lens assembly is to magnify the image appearing on the CRT faceplate and thereby project it onto a viewing screen, which is much larger than the faceplate of the CRT.
In a projection television, the image appearing on the CRT undergoes significant magnification. For example, typical CRTs used in projection televisions have a diameter of 3-6 inches. However, the images are projected onto a screen which typically has a size ranging from 48 to 60 inches or larger. In view of this substantial magnification, it is important that each of the CRTs provide maximum brightness or light intensity. Indeed, the need for light intensity is a reason why three separate CRTs are used in projection televisions. In order to maximize the intensity, each CRT is operated at maximum power to produce maximum light output at the faceplate. A consequence of operating at maximum power, together with the fact that each of the CRTs in projection television is contained within an enclosure, results in the generation of considerable heat within the enclosure. Consequently, each of the lens systems associated with each of the CRTs may undergo very considerable temperature change. Indeed, in projection television sets sold commercially and used residentially, it is not uncommon for the ambient temperature in the interior of the set to be elevated by 40.degree. C. to 45.degree. C. and some components by 90.degree. C.
As a result of the lens system being exposed to an elevated ambient temperature, a variety of distortions are caused in the lens system, which result from thermally induced expansion of the components of the system. One particularly troublesome distortion, well known and appreciated by those skilled in this art, is the thermally induced distortion of one or more of the lens elements, whereby the focus of the lens system will change as the ambient temperature increases. Thereby, after some hours of operation, the picture displayed on the screen will become blurred as a result of defocusing.
An illustrative, prior art projection television lens assembly 10 is shown in FIG. 1. As shown therein, a CRT 9 has a faceplate 11. The lens assembly 10 is affixed to the CRT 9 and includes a first lens element 18. As shown, the first lens element 18, which in the art is often referred to as a "C element" or "field flattener" and has a uniform thickness, is typically made of plastic and is convex to the CRT 9 and concave to the image side of the lens assembly 10. The periphery of the first lens element 18 is sandwiched between a coupler 19, which in turn is secured to the CRT 9, and focus mount 12. Typically, the space 13, defined by the first lens element 18, the faceplate 11 and the coupler 19, is filled with a liquid and defines a concave lens, which functions as a so-called field flattener. In addition, the liquid contained in the space 13 functions to conduct heat from the CRT 9 to the coupler 19, which, in turn, typically is provided with a plurality of fins (not shown) for maximizing heat transfer to the ambient environment.
Considering further the lens assembly 10 shown in FIG. 1, the lens elements 14, 15, 16 and 17 are all secured to a tubular member 20 referred to as a lens cell, which is received within a tubular member 12 referred to as a focus mount. The lens element 14 is often referred to as the so-called "A element". Similarly, the lens elements 15 and 16 are generally referred to as the "B elements". The lens element 17 may be categorized as either a so-called "B/C corrector" or the lens element 17 may be grouped together with the lens elements 15 and 16 and referred to, together, as the "B elements".
As may be seen in FIG. 1, the distances between the lens elements 14, 15, 16 and 17 are fixed because they are respectively secured to the tubular member 20. However, the spacing between the lens element 17 and the first lens element 18 may be adjusted by axially moving the lens cell 20 toward or away from the first lens element 18. When projection television sets are assembled, this axial movement is effected to focus the image on the screen. After the image is focused, the axial position of the tubular member 20 is then fixed with respect to the focus mount 12. An apparatus for achieving such adjustment is shown in U.S. Pat. No. 5,276,555.
After the lens assembly 10 is focused and the lens cell 20 is fixed with respect to the focus mount 12, the set is then supplied to the user. Thereupon, when operating the lens system is, as described above, subjected to an elevated ambient temperature. The lens element 16, as shown in FIG. 1, typically has the maximum positive power in the system. For that reason, and in anticipation of the high temperature to which it will be subjected, that element is typically made of glass, whereby it will undergo little distortion as a result of experiencing an elevated temperature. However, in contrast, the lens element 18 has significant negative power, is made of plastic and is quite thin. As a consequence, when subjected to an elevated ambient temperature and the heated liquid is in the space 13 for a long period of time, the first lens element 18 will distort and the distortion will be manifested as an increase of the radius of curvature of the lens and thereby a decrease in the negative power of that element. Without correction, the result of distortion of the first lens element 18 will be a loss of focus of the picture.
Although the invention disclosed in U.S. Pat. No. 4,236,790 provides a mechanism for compensating for lens distortion, the invention cannot be manually focused after assembly and can only be automatically focused after assembly has been completed. Further, the lens assembly disclosed in U.S. Pat. No. 4,525,745 teaches focusing of the lens assembly with movable parts, where a piston in a cylinder attached to the focus adjustment of the lens assembly to rotationally move within the cylinder, which in turn axially moves the lens housing, thus, achieving automatic focusing by both rotational and axial movement.
An object of this invention is to provide a construction and method of operation whereby, in a projection television lens assembly, there is automatic compensation for lens distortion which occurs as a result of temperature changes to which the assembly is exposed. Thereby, the focal length of the system is maintained and the loss of focus is either prevented or substantially reduced.
Another object is to provide a construction and method of operation of a PTV lens assembly whereby the distortion of a lens is automatically accommodated.